This invention relates to an optical glass which has optical constants of a refractive index (nd) of 1.88 or over and an Abbe number (ν d) within a range from 22 to 28 and is suitable for precision mold pressing A typical optical glass having a high refractive index is one which contains a large quantity of lead oxide. Since this type of optical glass is stable because of having high resistance to devitrification and has a low glass transition point (Tg), it has been used for precision mold pressing. For example, Japanese Patent Application Laid-open Publlication No. 1-308843 discloses an optical glass suited for precision mold pressing which comprises a large quantity of lead oxide.
Since, however, an environment in which precision mold pressing is performed is kept in a reducing atmosphere for preventing oxidization of the mold, reduced lead precipitates from the surface of a glass if the glass contains lead oxide and this precipitating lead is deposited on the surface of the mold with the result that difficulty arises in maintaining the precision surface of the mold. Further, lead oxide is detrimental to the environment and hence a lead-free optical glass has been desired for.
Japanese Patent Application Laid-open Publication No. 52-45612 discloses an optical glass of a SiO2—Nb2O5—RO—R2O system which is free of lead oxide. The refractive index (nd) of this optical glass, however, is within a range from 1.61 to 1.80.
Japanese Patent Application Laid-open Publication No. 2000-16830 discloses an optical glass which does not contain lead oxide, has a refractive index (nd) of 1.7 or over and has a yield point of 580° C. or below. In this publication, however, there is no disclosure of a specific example which has a refractive index (nd) of 1.88 or over.
It is an object of the present invention to eliminate the disadvantages of the prior art optical glasses and provide an optical glass suited for precision mold pressing which has a high refractive index (nd) of 1.88 or over and an Abbe number (ν d) within a range from 22 to 28 and also has a low glass transition point (Tg).